“We reached this district on the 2d of May. Vegetation was then forward, though there had been no rain. But it must be remembered that during the winter there is snow, and hence a good deal of moisture in the earth when the spring opens. The months of May and June were moderately warm. On the third of July the first rain fell. It then came in torrents, accompanied by hail, and lasted three or four hours. Many of our adobe houses were deluged with water, and the mountain-sides exhibited cataracts in every direction. The Arroyo, which passes through the village, and which furnishes barely water enough for our party and the animals, became so much swollen as to render it difficult to cross; and, by the time it had received the numerous mountain torrents, which fall into it within a mile from our camp, it became impassable for wagons, or even mules. The dry gullies became rapid streams, five or six feet deep, and sometimes fifty feet or more across. On this day, a party, in coming to the copper mines, from the plain below, where there had been no rain, found themselves suddenly in a region overflowing with water, so that their progress was arrested, and they were obliged to wait until the flood had subsided. After this we had occasional showers, during the months of July and August.”
The location of this mountain station is near the thirty-third degree of north latitude, while the northern limit of the equatorial belt, nowhere, except upon the mountain ranges and table-lands of Mexico, extends above 25°.
There, for the reason we have been considering, it does extend further north during July and August, in occasional showers, and in the vicinity of Mount Picacho, Mr. Bartlett met one of its mountain thunder-storms on the 13th of July, on his return south through Mexico, in latitude 32°, in the following year. (Personal Narrative, vol. ii. p. 285). These showers originated in strata of counter-trade, which had followed up along the eastern side of the mountains and not from strata which had crossed them and curved to the eastward, as is shown by the course of progression of the showers.
Let us look, in this connection, at a fact or two of great interest, though not directly connected with the point in hand. The southern limit of the extra-tropical belt in winter, on the Pacific coast of North America, is in the vicinity of San Diego, at about 32°. In summer, that limit is carried up above Astoria, which is in latitude 46° 11′—about 14°—yet New Mexico receives little if any rain in winter in the vicinity of Albuquerque, but does receive a limited supply of about seven inches in summer and autumn, five and a half inches of which falls in June, July, and August. Albuquerque is in latitude 35° 13′, below the southern summer limit of the extra-tropical belt, and north of the northern limit of the equatorial belt. This anomaly is explained by the extension west over northern New Mexico, of the extreme western edge of our concentrated counter-trade, by reason of its issuing further west from the equatorial belt in its northern extension in the summer months. This western edge, in curving to the east, north-east of New Mexico, covers the north-western States, Iowa, Minnesota, Wisconsin, etc., and furnishes them that great excess of summer precipitation which is a peculiarity of their climate; and its absence further east in winter, and the very great elevation of the Rocky Mountains and other ranges over which their ordinary counter-trade of that season curves, account for the absence of much precipitation and snow there, or over the valley of the Rio Grande in New Mexico, in winter.
We may now see, too, why the western coast and the Pacific region of the continent, below 45°, are so deficient in moisture. The S. E. trades, which arise from the western portion of the south Atlantic and the continent of South America, which, if it were not for the Andes chain, in their natural course, after passing the equatorial belt, would continue on to the north-west until they passed the limits of the N. E. trades, and curve in upon the western portion of our continent below 45°, and supply it bountifully with rain, are, in part, perhaps, diverted along the eastern side of those mountains to swell the volume of our counter-trade, and in part pass them, almost exhausted of their supply of moisture by their contiguous reciprocal action. Hence, too, the deficiency of precipitation at the base of the Andes, on the western side, and the peculiar and irregular character of the winds under the western lee of the Andean range. Baffling airs and bands of calms prevail on this portion of the Pacific, except where the mountains fall off, and then there is a westerly or south-westerly monsoon under the equatorial belt. Says Lieutenant Maury in his Charts, sixth edition, p. 731:
“The passage, under canvass, from Panama to California, as at present made, is the most tedious, uncertain, and vexatious that is known to navigators.
“My investigations have been carried far enough to show that at certain seasons of the year a vessel bound from Panama to California, must cross at least three, at some seasons four, such meetings of winds or bands of calms, before she can enter the region of the N. E. trades. Hence the tedious passage.”
Such will ever be the state of things on this continent and upon the eastern Pacific, so long as the S. E. counter-trades are compelled to pass over the mountain chain of South and Central America.
Again, if we examine carefully the belt or zone of extra-tropical rains, we shall find that the focus of greatest precipitation is considerably north of its southern limit, and that, other things being equal, this focus travels north in summer, and gives to higher latitudes their needed summer rains. This is very apparent upon the north-western portion of our continent, as the following table will show: